The present invention relates to a substrate having an electrode on a surface thereof and more particularly, to an improvement in a method for forming an electrode on a substrate.
In display devices such as liquid crystal display (LCD), electrochromic display (ECD), plasma display (PDP), and electroluminescent display (ELD) devices and in metal-insulator-semiconductor (MIS) solar cells, transparent electrodes are generally formed on the surface of a substrate made of glass or the like. A transparent film with low resistance can be obtained using a metal such as silver, but in order to ensure sufficient light transmissivity, it is necessary to form an extremely thin film having a thickness of approximately 10 nm. Difficulties, however, arise in the handling of such a thin film, as such a film can be easily damaged in subsequent steps such as in a step of patterning the electrodes. For these reasons, an oxide conductive material such as zinc oxide, indium oxide, or tin doped indium oxide (ITO), which although has a high resistivity in comparison to metal, has a high hardness and is not particularly susceptible to deterioration in function caused by oxide degradation, is used for the transparent electrodes formed on a substrate. In particular, films made of ITO are widely used because of the low resistivity thereof. In ITO, Sn4+, which substitutes for In3+ in oxide indium In2O3, generates a carrier electron. As is the case with indium oxide, the crystal structure is a cubic bixbyite structure. For the formation of an ITO film, sputtering, which enables the formation of a low resistance film on a substrate with a large area at a relatively low temperature, is generally employed. The characteristics of a film formed by sputtering vary according to the substrate temperature during formation. A film formed at a high temperature of approximately 200xc2x0 C. or higher is a so-called polycrystalline film made up of an aggregate of microcrystal. In such a film formed at a high temperature, microcrystals in each of certain regions orient in substantially the same direction, thereby forming domains. A temperature of 250xc2x0 C. has been determined to be the substrate temperature at which a film having both the highest possible transparency and the lowest possible resistance can be obtained. By contrast, a film formed at a low temperature is amorphous or has a structure mainly in an amorphous phase with crystal fine grains being dispersed therein. While an amorphous film has excellent etchability for the processing of electrodes, fundamental characteristics such as conductivity and transparency are inferior to those of a polycrystalline film. Chemical resistance and corrosion resistance are also inferior. In a film that is a mixture of the amorphous phase and the crystal phase, linearity of film patterning is poor because the etching speed for each phase differs greatly. In, for example, cases in which crystal grains are dispersed in an amorphous matrix, it is only the crystal phase that is not etched, remaining as residue and becoming a cause of defects. In addition, because most of the film is the amorphous phase, the film is inferior in terms of a variety of properties such as conductivity, transmissivity, chemical resistance, corrosion resistance, and durability.
For example, in Japanese Unexamined Patent Publication No. 4-48516, it is proposed that, for the transparent electrodes, an ITO film or an indium oxide film, each being oriented in a specified direction and having excellent patternability for etching, be employed. In the same publication, an amorphous film is proposed as an electrode material having excellent patternability for etching. While an amorphous film has excellent etchability, it is inferior in terms of fundamental characteristics such as conductivity and transparency. In Japanese Unexamined Patent Publication No. 8-94230, a randomly-oriented polycrystalline ITO film formed by sputtering at a substrate temperature of 180-350xc2x0 C. is proposed as a transparent conductive film having excellent etchability.
As in the above examples, polycrystalline films for electrodes have conventionally been formed by sputtering with the substrate temperature fixed at a high temperature of 200xc2x0 C. or higher. Thus, it was necessary to employ a substrate made of a material capable of withstanding a high temperature of 200xc2x0 C. or higher during formation of a film, such as a substrate made of glass.
In recent years, many attempts have been made to use a substrate made of a synthetic resin rather than a substrate made of glass, as a synthetic resin substrate has the advantages of being light weight and difficult to break. However, synthetic resin substrates can withstand at most a temperature of 180xc2x0 C. Therefore, it is not possible to form, on a surface of the substrate, a transparent conductive film made of an oxide such as ITO or zinc oxide at high temperatures of 200xc2x0 C. or higher. Namely, electrodes formed on a synthetic resin substrate had to be either a film in which the amorphous phase and the crystal phase are mixed or a film in the amorphous phase. In Japanese Unexamined Patent Publication No. 9-61836, a method is proposed wherein, by mixing H2O in a sputtering gas, an ITO thin film having excellent etchability and a low density of crystal grains dispersed in an amorphous matrix is formed on a synthetic resin substrate. This film, however, also is inferior to a polycrystalline film formed at a high temperature in terms of conductivity and transparency.
In Japanese Unexamined Patent Publication No. 5-346575, a method is proposed wherein resistance is improved by using vapor deposition to form an ITO film on a synthetic resin substrate at a temperature below the temperature at which the substrate undergoes a change in shape and calcining the film in air to adjust the degree of oxidization. However, with this method also, fundamental characteristics such as resistivity and transmissivity are insufficient in the resulting film.
The temperature of heating is limited, not only in cases where substrates made of synthetic resin are used, but also in cases where a color filter layer and a light-emitting layer made of an organic material are provided on a same substrate. For example, a transparent conductive film for use as the electrodes of a super twisted nematic (STN) mode color LCD is formed on a color filter made of organic material. The substrate temperature during the forming of this transparent conductive film is limited to approximately 200xc2x0 C. or less due to the presence of the color filter material.
Thus, there has been a need for a method of forming, at a low temperature, a conductive film that has excellent resistivity, transmissivity, etchability, and the like.
In addition, because of the difference in the thermal expansion coefficient of a substrate and a conductive thin film formed on the surface of the substrate, the substrate is susceptible to bend. When bend arises in a substrate, precision in the processing of the formed conductive thin film into electrodes is reduced. The higher the temperature during formation of the conductive thin film, the larger the bend becomes. In other words, when the film is formed at a low temperature, while bend in the substrate is minimal, a film with sufficient characteristics cannot be obtained. In amorphous silicon (a-Si) solar cells in which a transparent conductive film, an amorphous silicon layer, and an aluminum electrode layer are stacked on a substrate, in organic ELDs in which a pair of electrode layers and an organic light-emitting layer sandwiched therebetween are stacked on a substrate, and the like, precision in the processing of other layers stacked on the conductive thin films suffers. In any case, in order to obtain a high wiring density and a stable internal resistance, it is absolutely necessary that such a bend be reduced. In particular, in a-Si solar cells, which are set up outside, good reliability and durability are required, making bend a serious problem.
Synthetic resin substrates are more susceptible to bend than are glass substrates. For this reason, the formation of conductive thin films on synthetic resin substrates has conventionally been carried out in the relatively low range of from room temperature to 150xc2x0 C. In order to meet the demand for a reduction in the size, a reduction in the weight, and an improvement in light transmissivity of display panels, it is necessary that the thickness of the substrates be reduced. At the same time, in order to reduce the internal resistance of a circuit, it is necessary that the thickness of conductive films for electrodes and the like be increased. Thus, as reduction in the size and improvement in the performance of devices progress, the more serious a problem bend in substrates and fractures and the like that arise along with bend become.
In Japanese Unexamined Patent Publication No. 2000-222944, a method of forming a low stress and low resistance ITO film is proposed with the aim of preventing fractures in the thin film. In this method, under the condition that the substrate temperature be approximately 200xc2x0 C., a polycrystalline film made up of an aggregate of polygonal column-shaped crystal grains is formed by ion plating with arc discharge plasma. Although this method is effective for preventing fractures in the thin film, it is not effective for suppressing the bend in a substrate that arises with heating at a high temperature. It is also difficult to use this method for forming a film at a low temperature on a substrate made of synthetic resin. Thus, there has been a need for a method of forming, at a low temperature, a conductive thin film having excellent resistivity, transmissivity, etchability, and the like, that does not also bring about a change in the shape of the substrate.
In order to overcome the foregoing problems, it is an object of the present invention to provide a method of forming, at a low temperature, a conductive film that is especially useful when utilizing a synthetic resin substrate and that is excellent in terms of a variety of properties such as resistivity, transmissivity, chemical resistance, corrosion resistance, patternability, and adhesion. It is another object of the present invention to provide a method of forming a conductive film wherein the forming of the film on a substrate renders a substrate in which bend is small.
According to a method of producing a substrate with an electrode of the present invention, an oxide conducive film composed of an amorphous material or mainly composed of an amorphous material is formed on a substrate at a temperature equal to or less than the crystallization temperature of the film, and subsequently the formed oxide conductive film is crystallized by heating. The oxide conductive film is processed into the shape of an electrode either before or after the crystallization, according to necessity.
According to the present invention, crystals in the film are grown in a step subsequent to film formation, making it possible to form the film at a low temperature. The amount of stress that arises in the formed film is dependant on the degree of change in shape of the substrate during formation of the film, i.e., the substrate temperature. Formation of the film at an even lower temperature is effective for suppressing bend in the substrate. Preferably, the film is formed at a temperature of 150xc2x0 C. or less, because a substrate with an electrode having a small bend, excellent adhesion, and high reliability can thereby be obtained, owing to the fact that internal stress in the transparent conductive film caused by the change in shape of the substrate with heating is small. In particular, when a synthetic resin substrate, which is susceptible to change in shape, is used, it is possible to significantly reduce bend in the substrate by forming the film at normal temperature.
The crystallization of the oxide conductive film is carried out by heating the oxide conductive film. It is not always necessary to carry out crystallization at a temperature equal to or higher than the crystallization temperature. From the standpoint of the amount of time required for the treatment, the crystallization is preferably carried out at a temperature equal to or less than the maximum temperature the substrate can withstand, such as the glass transition temperature of the substrate, specifically a temperature in the range of 150xc2x0 C.-200xc2x0 C., though it is also possible to carry out the crystallization at a temperature less than this temperature range. It is, of course, preferable that the temperature be equal to or higher than the substrate temperature during the formation of the film. The step of crystallizing the oxide conductive film may be carried out under an atmosphere containing oxygen or an atmosphere not containing oxygen.
The oxide conductive film formed on the substrate is made of, for example, indium oxide or indium oxide having a portion substituted by tin (ITO). In order to effectively crystallize the oxide conductive film at a low temperature, the film is made to have a tin oxide content of less than 5% by weight. When the added amount of tin is made small, the crystallization temperature is reduced, thereby making it possible to even more effectively carry out the crystallization treatment at a low temperature. In conventional production of a thin film by sputtering, a low tin oxide content results in an increased resistivity, but in the present invention, crystallization of the film is brought about by a treatment carried out after film formation, making it possible to obtain an ITO film having a low resistivity even when the tin oxide content is low.
The size of a single crystal grain after crystallization is determined by the crystal state of the film immediately after formation of the film on the substrate, in other words, by the size of the fine crystal grains dispersed in the amorphous matrix and the dispersion density. In order to form a polycrystalline film that has an average grain size of 20-300 nm and random orientation and that has excellent transmissivity, conductivity, etchability, and the like, and low internal stress, it is desirable that the oxide conductive film have a structure wherein, at the stage immediately after the formation of the film on the substrate, crystal grains having an average grain size of 200 nm or less are dispersed in an amorphous matrix. Modification in the size of crystal grains dispersed in the amorphous matrix and the dispersion density is made possible by varying conditions such as substrate temperature, gas pressure, and production speed during formation of the film. These conditions are determined according to electrode material.
The present invention is particularly useful for the production of a substrate with an electrode that utilizes a synthetic resin substrate with which formation of a film at a low temperature is required.
It is preferable that an undercoat layer made of an organic material be provided on a surface of a substrate to relieve stress caused by the difference in the coefficient of thermal expansion between the substrate and the electrode formed thereon.
Forming a transparent coating film containing a synthetic resin on the surface of the completed electrode is effective for suppressing bend in a substrate. As long as the volume resistance of this transparent coating film is 102-1012 xcexa9xc2x7cm or less, damage to electrode function is substantially nonexistent. Because a substrate provided with a transparent coating film is not susceptible to bend even with a change in temperature, use of a substrate of the present invention for substrates of display panels such as LCD and organic ELD panels makes a high quality display in a wide temperature range possible. It should be noted that providing the transparent coating film so as to cover the electrode prevents damage to the electrode in subsequent steps, thereby making it possible to use conductive materials other than oxide conductive materials, such as metal, for the electrode material. In particular, when an extremely thin metal film is used for a transparent electrode or a translucent electrode, the film formed on the surface of this metal film serves as a protection film for preventing damage to the metal film in subsequent steps.
For example, a resist is used for the transparent coating film that is formed on the surface of the electrode. A resist layer containing a light-curing resin is formed on the completed conductive film, regions corresponding to an electrode pattern for processing the oxide conductive film are cured by exposure to thus form the transparent coating film, and the oxide conductive film is etched using the cured transparent coating film as a resist. The etching thus makes it possible to form a transparent coating film that covers the electrode and has a shape that substantially conforms to that of the electrode, simultaneously with processing the conductive film into electrode. In this way, it is not necessary to add an additional step in order to form a transparent coating film. A step of removing the resist may also be omitted.
Use of a light-curing resin in which conductive powder is dispersed makes it possible to obtain a transparent conductive film having a desired volume resistance.
When the thickness of the transparent coating film is 0.5-5 xcexcm, a good display is possible even if a substrate having such a film formed thereon is utilized as the substrates for a liquid crystal panel.
A substrate with an electrode of the present invention comprises a substrate and an electrode made of an oxide conductive film disposed on the substrate, the electrode being composed of a polycrystalline film having an average grain size of 25 nm or larger, preferably 40 nm or larger, where a crystal is the largest constitutional unit having a boundary identifiable by surface observation. In other words, the polycrystalline film does not have so-called domains, aggregates of crystal having a substantially aligned orientation direction. In such a film, because stress arising in the conductive film is small compared to conventional transparent conductive films having grains, the film is not particularly susceptible to cracks and fractures caused by stress. Bend in the substrate is also small.
In particular, in comparison to an amorphous film, a conductive film with which a clear diffraction peak has been confirmed by X-ray analysis, verifying an average crystal grain size of 20 nm or larger, has a low resistance, excellent transmissivity, excellent chemical resistance, and excellent corrosion resistance. In addition, the film does not have uneven distortion, and thus has excellent linearity in patterning. The average grain size of the crystals is preferably 300 nm or less.
When the thickness of the electrodes is as thin as less than 15 nm, the progress of the crystallization by heating is particularly slow. On the other hand, when crystallization is carried out at a high temperature on a thick film having a thickness that exceeds 1,500 nm, breaks and fractures are more likely to arise. Furthermore, such crystallization deepens corrugations in the surface of the film. Therefore, in order to ensure sufficient conductivity while more effectively preventing fractures in the film and bend in the substrate, it is preferable that the thickness of the formed oxide conductive film be 15-1,500 nm, and more preferably, 50-500 nm. When the height variation of the surface of the electrode is 20 nm or less, a device with stable characteristics can be obtained. For the oxide conductive film, ITO can be used, preferably having a tin oxide content of less than 5% by weight.
Coating the surface of the electrode by disposing a transparent coating film that contains a synthetic resin and has a volume resistance in the range of 102-1012 xcexa9xc2x7cm is effective in protecting the electrode. When the substrate is made of a synthetic resin, this coating of the substrate surface makes is possible to suppress change in the shape of the substrate caused by thermal expansion, as the electrode is then sandwiched between synthetic resin having similar coefficients of thermal expansion.
In order to function as a protection film for an easily damaged metal electrode, it is preferable that the thickness be 0.5 xcexcm or greater. When used in a display panel, the thickness is preferably 5 xcexcm or less so that image quality is not affected. As for volume resistance, when film thickness is 5 xcexcm, it is preferable that volume resistance be 1012 xcexa9xc2x7cm or less. In the case of using a metal film for a transparent electrode, thickness of the metal film is preferably 20 nm or less, and more preferably, 10 nm.